Natural gas is employed as an alternative fuel for vehicles to replace conventional liquid fuels like gasoline and diesel. There are a number of factors motivating the use of natural gas, of which, two are cost and emissions. On an energy equivalent basis natural gas is less expensive than petroleum based fuels. The price of crude oil continues to increase as production continues to outpace discoveries of new oil reserves. In contrast, the reserves of natural gas continue to increase as production lags behind the discovery of new reserves keeping the price well below oil. Engines fuelled with natural gas produce fewer emissions than engines fuelled with either gasoline or diesel. Due to ever more stringent emission standards engine manufacturers are looking to natural gas to meet these new standards.
The refueling infrastructure for natural gas vehicles is not as extensive as that for conventional liquid fuels; this is a factor that influences adoption of natural gas vehicles especially for consumer automobiles. Access to refueling stations is currently limited to urban areas and main transportation corridors which limits the range of travelling and requires vehicle operators to make planned refueling trips. For these reasons natural gas has had greater adoption in the heavy duty diesel trucking industry since these vehicles typically operate along the natural gas corridor and/or use private refueling facilities. However, due to the above factors motivating the use of natural gas, automobile manufacturers are beginning to integrate natural gas fuel systems alongside existing gasoline fuel systems and to adapt internal combustion engines to be fuelled with more than one fuel, these being referred to in this disclosure as “multi-fuel engines”.
In this disclosure, the terms “natural gas” and “gas” are used interchangeably and understood to be preferred examples of a gaseous fuel, but that other gaseous fuels such as ethane, methane, propane, biogas, landfill gas, dimethyl ether, hydrogen and mixtures thereof could also be employed instead of natural gas. In one such multi-fuel engine there is a direct injection fuel system which introduces liquid fuel directly into combustion chambers, and a natural gas port injection fuel system which introduces natural gas into the intake air upstream of intake valves. In this engine liquid fuel remains dormant in direct fuel injectors that are not being actuated when operating in a port injection natural gas fuelled mode. In this mode, because the nozzles of the direct fuel injectors are located in the combustion chamber it is possible that heat from combustion of port injected fuel elevates the temperature of the liquid fuel inside the direct fuel injectors above a threshold temperature such that the injectors are damaged or carbon deposits begin form. The formation of these carbon deposits leads to fouling of the direct fuel injectors impacting the performance of liquid fuel injection.
In another engine system there are both liquid fuel direct and port fuel injection systems. Depending upon the current operating mode the engine can be fuelled with either the direct or port fuel injection system or both simultaneously. The liquid fuel that is used to fuel the engine and delivered to the direct and port fuel injection systems can be the same fuel or different fuels if the engine is a multi-fuel engine. For example, when the engine starts it is advantageous to fuel from the direct injection system in a stratified charge mode, and when under high load or speed the engine can fuel from the port injection system in a premixed mode. Direct fuel injectors can become fouled when liquid fuel remains dormant inside while operating the engine with fuel from the port injection system.
U.S. Pat. No. 7,853,397, issued Dec. 14, 2010 to Pott et al. (the '397 patent), discloses a method of operating an internal combustion engine that operates with carburetor fuel, injected through a high pressure direct injector, and with gas fuel introduced into the intake air manifold or ports. In gas operation there is the risk that the high pressure direct injectors heat up due to the lack of through-put of liquid fuel and are subsequently damaged or the fuel located inside forms deposits which have an adverse effect on injector behavior. To avoid these problems, a load characteristic of the high pressure fuel injector is determined and if this load is above a limit value then switchover to carburetor fuel operation is performed, or carburetor fuel operation is hooked into gas operation such that the fuel in the high pressure injector is purged and the injector is cooled. Based on engine temperatures (operating parameters) a thermal load upon the fuel injector is retrieved from a weighing characteristic map, which is integrated over time to determine the load characteristic value. The method of the '397 patent does not determine the temperature of the high pressure injector, but instead determines stored energy representing the empirical thermal load upon the injector. As a result, during gas operation carburetor fuel can be consumed unnecessarily based on the stored energy value even though the temperature of the fuel injector is below a critical value above which deposits begin to form. The method of the '397 patents determines the thermal load upon the high pressure injector during gas operation only, and does not continuously determine the thermal load for all fuelling modes (gas operation, carburetor fuel operation and mixed fuel operation). That is, during gas operation when it is determined that the thermal load is above the limit value, carburetor fuel is flowed through the high pressure fuel injector to purge fuel and cool the injector. The amount of liquid fuel flowed through the injector is based on a predetermined minimum volume, which is expected to cool the injector, instead of the volume required to reduce the temperature of the fuel injector below the critical value at which deposits begin to form. Again, during gas operation, this results in unnecessary and increased carburetor fuel operation.
The state of the art is lacking in techniques for protecting direct injectors in multi-fuel system engines that reduce or minimize the amount of directly injected fuel that is introduced to cool the direct injectors. Accordingly, for engines that can be fuelled through a direct injection system as well as by means of another fuel system there is a need for an improved method of protecting the direct injector when operating with the other fuel system.